build: bootstrap on Plan 9

This change implements the requirement of
old Go to build new Go on Plan 9. Also fix
the build of the new cmd/dist written in Go.

This is similar to the make.bash change in
CL 2470, but applied to make.rc for Plan 9.

Change-Id: Ifd9a3bd8658e2cee6f92b4c7f29ce86ee2a93c53
Reviewed-on: https://go-review.googlesource.com/2662Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/sys_default.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !windows,!plan9
+// +build !windows
 
 package main
 

src/make.rc

@@ -3,6 +3,8 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
+# See golang.org/s/go15bootstrap for an overview of the build process.
+
 # Environment variables that control make.rc:
 #
 # GOROOT_FINAL: The expected final Go root, baked into binaries.
@@ -45,17 +47,18 @@ rm -f ./runtime/runtime_defs.go
 # Determine the host compiler toolchain.
 eval `{grep '^(CC|LD|O)=' /$objtype/mkfile}
 
-echo '# Building C bootstrap tool.'
+echo '# Building Go bootstrap tool.'
 echo cmd/dist
 GOROOT = `{cd .. && pwd}
-if(! ~ $#GOROOT_FINAL 1)
-	GOROOT_FINAL = $GOROOT
-DEFGOROOT='-DGOROOT_FINAL="'$GOROOT_FINAL'"'
-
-for(i in cmd/dist/*.c)
-	$CC -FTVwp+ -DPLAN9 $DEFGOROOT $i
-$LD -o cmd/dist/dist *.$O
-rm *.$O
+if(! ~ $#GOROOT_BOOTSTRAP 1)
+	GOROOT_BOOTSTRAP = $home/go1.4
+if(! test -x $GOROOT_BOOTSTRAP/bin/go){
+	echo 'ERROR: Cannot find '$GOROOT_BOOTSTRAP'/bin/go.' >[1=2]
+	echo 'Set $GOROOT_BOOTSTRAP to a working Go tree >= Go 1.4.' >[1=2]
+	exit bootstrap
+}
+rm -f cmd/dist/dist
+GOROOT=$GOROOT_BOOTSTRAP $GOROOT_BOOTSTRAP/bin/go build -o cmd/dist/dist ./cmd/dist
 
 eval `{./cmd/dist/dist env -9}
 echo